A commercially available electrolytic capacitor as a rule is made of a porous metal electrode, an oxide layer serving as a dielectric on the metal surface, an electrically conductive material, usually a solid, which is introduced into the porous structure, an outer electrode (contacting), such as e.g. a silver layer, and further electrical contacts and an encapsulation. An electrolytic capacitor which is frequently used is the tantalum electrolytic capacitor, the anode electrode of which is made of the valve metal tantalum, on which a uniform, dielectric layer of tantalum pentoxide has been generated by anodic oxidation (also called “forming”). A liquid or solid electrolyte forms the cathode of the capacitor. Aluminium capacitors in which the anode electrode is made of the valve metal aluminium, on which a uniform, electrically insulating aluminium oxide layer is generated as the dielectric by anodic oxidation, are furthermore frequently employed. Here also, a liquid electrolyte or a solid electrolyte forms the cathode of the capacitor. The aluminium capacitors are usually constructed as wound- or stacked-type capacitors.
π-conjugated polymers are particularly suitable as solid electrolytes in the capacitors described above because of their high electrical conductivity. π-conjugated polymers are also called conductive polymers or synthetic metals. They are increasingly gaining economic importance, since polymers have advantages over metals with respect to processability, weight and targeted adjustment of properties by chemical modification. Examples of known n-conjugated polymers are polypyrroles, polythiophenes, polyanilines, polyacetylenes, polyphenylenes and poly(p-phenylene-vinylenes), a particularly important polythiophene used industrially being poly(3,4-ethylenedioxythiophene) (PEDOT), since it has a very high conductivity in its oxidized form.
The solid electrolytes based on conductive polymers can be applied to the oxide layer in various ways. EP-A-0 340 512 thus describes, for example, the production of a solid electrolyte from 3,4-ethylenedioxythiophene and the use thereof in electrolytic capacitors. According to the teaching of this publication, 3,4-ethylenedioxythiophene is polymerized on to the oxide layer in situ.
The disadvantage of the production of solid electrolytic capacitors using an in situ polymerization is however, amongst others, the complexity of the process. Thus, a polymerization process which in each case includes the process steps of impregnation, polymerization and washing as a rule lasts several hours. Under certain circumstances, readily flammable or toxic solvents must also be employed here. A further disadvantage of the in situ process for the production of solid electrolytic capacitors is that as a rule anions of the oxidizing agent or, where appropriate, other monomeric anions serve as counter-ions for the conductive polymer. Because of their small size, however, these are not bonded to the polymer in a sufficiently stable manner. As a result, diffusion of the counter-ions and therefore an increase in the equivalent series resistance (ESR) of the capacitor may occur, especially at elevated use temperatures of the capacitor. The alternative use of high molecular weight polymeric counter-ions in the chemical in situ polymerization does not lead to sufficiently conductive films and therefore does not lead to low ESR values.
In addition to the in situ polymerization described above, processes for the production of solid electrolytes in capacitors in which a dispersion comprising the already polymerized thiophene and a polyanion as a counter-ion, for example the PEDOT/PSS dispersions known from the prior art, is applied to the oxide layer and the dispersing agent is then removed by evaporation are also known from the prior art.
The disadvantage of the capacitors obtained by employing such dispersions, however, is inter alia that they have on the one hand a comparatively low capacitance and on the other hand unsatisfactory low temperature properties. “Low temperature properties” of a capacitor in this context are understood as meaning the influencing of the electrical characteristic values thereof, such as, for example, the capacitance, the equivalent series resistance, the breakdown voltage or the residual current, but in particular the influencing of the capacitance, at low temperatures, in particular at temperatures down to below −60° C.